[libc++][format] Fix precision parser conformance.

@CaseyCarter reported that the tests for the std-format-spec rejects leading
zeroes for precision, which the Standard does not require. The Standard allows
them. Only for precision, not for the width or an arg-id.

Fixes the precision parser and adds some test for the arg-id since they
were missing.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D116381